1. Field of the Invention
The present invention relates to an image forming apparatus capable of accentuating a particular part of an image of a document by rendering the background of the particular part in a color different from a color of the background of the other part. More particularly, the present invention relates to an image forming apparatus which, in the event that a solid image representative of the background of the particular part is to be formed, controls the amount of toner of the different color to thereby form the solid image with constant density at all times.
2. Discussion of the Background
There has been proposed an electrophotographic copier, facsimile apparatus, laser printer or similar image forming apparatus which is operable in a so-called undercolor mode for rendering the background of a particular area of an image of a document in the form of a solid image which is different in color from the background of the other area of the image for the purpose of accentuating the particular part. With an electrophotographic copier, for example, it has been customary to implement such an undercolor mode by any of two different types systems: a constant or non-controlled supply type system in which an amount of toner supply per copy for forming a solid image is determined beforehand on the basis of an average image occupation ratio of documents of ordinary use so as to supply the same amount of toner without exception, and a controlled supply type system in which a reference pattern of predetermined density is formed on a drum and the amount of toner supply is controlled by sensing the density. For example, a solid image may be formed by red toner in a particular part or region of a document image so as to accentuate the information lying in that part in distinction from the other information.
A problem with the constant supply type system stated above is as follows. When a solid image representative of the background of a particular region of a document image has a relatively large area, the density of the solid image is lowered because the amount of toner supply is constant and not variable. Conversely, when the area of the solid image is relatively small, the density of the solid image is increased for the same reason resulting in the toner being scattered around or even the information in the particular part being practically smeared out. On the other hand, the controlled supply type system cannot form a solid image without resorting to a complicated sequence of steps, i.e., causing optics to scan a reference pattern to expose a photoconductive drum imagewise, sensing the density of the reference pattern produced on the drum, selecting an amount of toner supply based on the sensed density, turning off a lamp of the optics, and turning on an eraser to erase an electrostatic charge which has been deposited in the area other than the solid image. Further, such a procedure slows down the copying operations.